Estimation of soil hydraulic properties of a sandy soil using capacitance probes and guelph permeameter1

Soil hydraulic conductivity (K)-water content (θ)-pressure head (h) relationships (K-θ-h) are key parameters for crop growth, irrigation, drainage, and modeling water flow and chemical transport through the soil. Several laboratory methods have been used to determine these parameters and to extrapolate to field conditions. However, it is essential to determine these parameters under field conditions to minimize the effect of spatial variability. The objective of this study was to use the Instantaneous Profile method in combination with capacitance probes, tensiometers, the Guelph permeameter, and the van Genuchten hydraulic functions to determine the hydraulic conductivity-water content-pressure head relationships of a Candler fine sand (hyperthermic, uncoated, Typic Quartzipsamments) soil. The soil was flooded for more than 2 h to achieve a constant water content through the profile and was then covered with a plastic sheet to prevent evaporative loss or addition of rain water. The capacitance probe and tensiometer readings were taken simultaneously at 0.1-, 0.2-, 0.4-, 0.7-, and 1.1-m depths. Both water content and hydraulic conductivity values decreased substantially as water redistributed through the soil profile after saturation. Saturated hydraulic conductivity, measured with the Guelph permeameter for the five soil depths, varied between 6.1 and 10.0 m day. More than 50% of the soil water was drained from the soil profile within 8 to 10 h following saturation. The analytical RETC (RETension Curve) model was used to extend soil water release curves beyond the limited range of tensiometer suction measurements. Results of this study demonstrate that the Sentek capacitance probe is a practical tool that can be used in combination with the Guelph permeameter and analytical fitting software (RETC) to determine in-situ soil water characteristic curves, saturated hydraulic conductivity, and k(θ) at difference soil depths.